The answer to 'drainage system yellowing' is that it is typically caused by mineral buildup or bacterial growth in the drainage pipes. Symptoms include discoloration, slow drainage, and foul odors. Solutions may involve flushing the system, treating with chemical cleaners, or replacing damaged pipes.
Homeowners can often address minor drainage system issues through DIY maintenance, but more complex problems may require the expertise of a professional landscaping or plumbing contractor. It's important to identify and address the root cause to prevent further damage and ensure proper drainage.